A recent post from the Google Base blog informed sellers that they can submit tax and shipping information. "We'll soon be displaying this information on Google Product Search to provide buyers with more accurate price information, so we strongly encourage all merchants to start submitting it."
In case you are wondering, Google Base is a low-profile service that lets you upload structured data. Google uses the data about products in Google Product Search and the real estate listings as a refinement option in Google Maps and the main search results, but I find it surprising that Google doesn't use the data in expanded snippets.